Multi-dimensional Roles of Ketone Bodies in Fuel Metabolism, Signaling, and Therapeutics

Ketone body metabolism is a central node in physiological homeostasis. In this review, we discuss how ketones serve discrete fine-tuning metabolic roles that optimize organ and organism performance in varying nutrient states and protect from inflammation and injury in multiple organ systems. Traditionally viewed as metabolic substrates enlisted only in carbohydrate restriction, observations underscore the importance of ketone bodies as vital metabolic and signaling mediators when carbohydrates are abundant. Complementing a repertoire of known therapeutic options for diseases of the nervous system, prospective roles for ketone bodies in cancer have arisen, as have intriguing protective roles in heart and liver, opening therapeutic options in obesity-related and cardiovascular disease. Controversies in ketone metabolism and signaling are discussed to reconcile classical dogma with contemporary observations. Ketone body metabolism is a central node in physiological homeostasis. In this review, we discuss how ketones serve discrete fine-tuning metabolic roles that optimize organ and organism performance in varying nutrient states and protect from inflammation and injury in multiple organ systems. Traditionally viewed as metabolic substrates enlisted only in carbohydrate restriction, observations underscore the importance of ketone bodies as vital metabolic and signaling mediators when carbohydrates are abundant. Complementing a repertoire of known therapeutic options for diseases of the nervous system, prospective roles for ketone bodies in cancer have arisen, as have intriguing protective roles in heart and liver, opening therapeutic options in obesity-related and cardiovascular disease. Controversies in ketone metabolism and signaling are discussed to reconcile classical dogma with contemporary observations. Ketone bodies are a vital alternative metabolic fuel source for all domains of life, eukarya, bacteria, and archaea (Aneja et al., 2002Aneja P. 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In mammals, ketone bodies are produced predominantly in the liver from FAO-derived acetyl-coenzyme A (CoA), and they are transported to extrahepatic tissues for terminal oxidation. This physiology provides an alternative fuel that is augmented by relatively brief periods of fasting, which increases fatty acid availability and diminishes carbohydrate availability (Cahill, 2006Cahill Jr., G.F. Fuel metabolism in starvation.Annu. Rev. Nutr. 2006; 26: 1-22Crossref PubMed Scopus (297) Google Scholar, McGarry and Foster, 1980McGarry J.D. Foster D.W. Regulation of hepatic fatty acid oxidation and ketone body production.Annu. Rev. Biochem. 1980; 49: 395-420Crossref PubMed Google Scholar, Robinson and Williamson, 1980Robinson A.M. Williamson D.H. Physiological roles of ketone bodies as substrates and signals in mammalian tissues.Physiol. Rev. 1980; 60: 143-187Crossref PubMed Scopus (657) Google Scholar). Ketone body oxidation becomes a significant contributor to overall energy mammalian metabolism within extrahepatic tissues in myriad physiological states, including fasting, starvation, the neonatal period, post-exercise, pregnancy, and adherence to low-carbohydrate diets. Circulating total ketone body concentrations in healthy adult humans normally exhibit circadian oscillations between approximately 100 and 250 μM, rise to ∼1 mM after prolonged exercise or 24 hr of fasting, and can accumulate to as high as 20 mM in pathological states like diabetic ketoacidosis (Cahill, 2006Cahill Jr., G.F. Fuel metabolism in starvation.Annu. Rev. Nutr. 2006; 26: 1-22Crossref PubMed Scopus (297) Google Scholar, Johnson et al., 1969bJohnson R.H. Walton J.L. Krebs H.A. Williamson D.H. Post-exercise ketosis.Lancet. 1969; 2: 1383-1385Abstract PubMed Google Scholar, Koeslag et al., 1980Koeslag J.H. Noakes T.D. Sloan A.W. Post-exercise ketosis.J. 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Rev. 1989; 5: 247-270Crossref PubMed Google Scholar), which contribute between 5% and 20% of total energy expenditure in fed, fasted, and starved states (Balasse et al., 1978Balasse E.O. Fery F. Neef M.A. Changes induced by exercise in rates of turnover and oxidation of ketone bodies in fasting man.J. Appl. Physiol. 1978; 44: 5-11Crossref PubMed Google Scholar, Cox et al., 2016Cox P.J. Kirk T. Ashmore T. Willerton K. Evans R. Smith A. Murray A.J. Stubbs B. West J. McLure S.W. et al.Nutritional ketosis alters fuel preference and thereby endurance performance in athletes.Cell Metab. 2016; 24: 256-268Abstract Full Text Full Text PDF PubMed Google Scholar). Studies highlight imperative roles for ketone bodies in mammalian cell metabolism, homeostasis, and signaling under a variety of physiological and pathological states. Apart from serving as energy fuels for extrahepatic tissues like brain, heart, or skeletal muscle, ketone bodies play pivotal roles as signaling mediators, drivers of protein post-translational modification (PTM), and modulators of inflammation and oxidative stress. In this review, we provide both classical and modern views of the pleiotropic roles of ketone bodies and their metabolism. The rate of hepatic ketogenesis is governed by an orchestrated series of physiological and biochemical transformations of fat. Primary regulators include lipolysis of fatty acids from triacylglycerols, transport to and across the hepatocyte plasma membrane, transport into mitochondria via carnitine palmitoyltransferase 1 (CPT1), the β-oxidation spiral, TCA cycle activity and intermediate concentrations, redox potential, and the hormonal regulators of these processes, predominantly glucagon and insulin (reviewed in Arias et al., 1995Arias G. Matas R. Asins G. Hegardt F.G. Serra D. 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Classically ketogenesis is viewed as a spillover pathway, in which β-oxidation-derived acetyl-CoA exceeds citrate synthase activity and/or oxaloacetate availability for condensation to form citrate. Three-carbon intermediates exhibit anti-ketogenic activity, presumably due to their ability to expand the oxaloacetate pool for acetyl-CoA consumption, but hepatic acetyl-CoA concentration alone does not determine ketogenic rate (Foster, 1967Foster D.W. Studies in the ketosis of fasting.J. Clin. Invest. 1967; 46: 1283-1296Crossref PubMed Google Scholar, Rawat and Menahan, 1975Rawat A.K. Menahan L.A. Antiketogenic action of fructose, glyceraldehyde, and sorbitol in the rat in vivo.Diabetes. 1975; 24: 926-932Crossref PubMed Google Scholar, Williamson et al., 1969Williamson D.H. Veloso D. Ellington E.V. Krebs H.A. Changes in the concentrations of hepatic metabolites on administration of dihydroxyacetone or glycerol to starved rats and their relationship to the control of ketogenesis.Biochem. J. 1969; 114: 575-584Crossref PubMed Google Scholar). The regulation of ketogenesis by hormonal, transcriptional, and post-translational events together supports the notion that the molecular mechanisms that fine-tune the ketogenic rate remain incompletely understood (see Regulation of HMGCS2 and SCOT/OXCT1). Ketogenesis occurs primarily in hepatic mitochondrial matrix at rates proportional to total fat oxidation. After transport of acyl chains across the mitochondrial membranes and β-oxidation, the mitochondrial isoform of 3-hydroxymethylglutaryl-CoA synthase (HMGCS2) catalyzes the fate-committing condensation of acetoacetyl-CoA (AcAc-CoA) and acetyl-CoA to generate hydroxymethylglutaryl (HMG)-CoA (Figure 1A). Hydroxymethylglutaryl-coenzyme A lyase (HMGCL) cleaves HMG-CoA to liberate acetyl-CoA and acetoacetate (AcAc), and the latter is reduced to D-β-hydroxybutyrate (D-βOHB) by phosphatidylcholine-dependent mitochondrial D-βOHB dehydrogenase (BDH1) in a NAD+/NADH-coupled near-equilibrium reaction (Bock and Fleischer, 1975Bock H. Fleischer S. Preparation of a homogeneous soluble D-beta-hydroxybutyrate apodehydrogenase from mitochondria.J. Biol. Chem. 1975; 250: 5774-5781PubMed Google Scholar, Lehninger et al., 1960Lehninger A.L. Sudduth H.C. Wise J.B. D-beta-hydroxybutyric dehydrogenase of muitochondria.J. Biol. Chem. 1960; 235: 2450-2455Abstract Full Text PDF PubMed Google Scholar). The BDH1 equilibrium constant favors D-βOHB production, but the ratio of AcAc/D-βOHB ketone bodies is directly proportional to the mitochondrial NAD+/NADH ratio; thus, BDH1 oxidoreductase activity modulates mitochondrial redox potential (Krebs et al., 1969Krebs H.A. Wallace P.G. Hems R. Freedland R.A. 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The mechanisms through which ketone bodies are transported across the mitochondrial inner membrane are not known, but AcAc/D-βOHB is released from cells via monocarboxylate transporters (in mammals, MCT1 and MCT2, also known as solute carrier 16A family members 1 and 7) and transported in the circulation to extrahepatic tissues for terminal oxidation (Cotter et al., 2011Cotter D.G. d’Avignon D.A. Wentz A.E. Weber M.L. Crawford P.A. Obligate role for ketone body oxidation in neonatal metabolic homeostasis.J. Biol. Chem. 2011; 286: 6902-6910Crossref PubMed Scopus (34) Google Scholar, Halestrap and Wilson, 2012Halestrap A.P. Wilson M.C. The monocarboxylate transporter family—role and regulation.IUBMB Life. 2012; 64: 109-119Crossref PubMed Scopus (189) Google Scholar, Halestrap, 2012Halestrap A.P. The monocarboxylate transporter family—structure and functional characterization.IUBMB Life. 2012; 64: 1-9Crossref PubMed Scopus (174) Google Scholar, Hugo et al., 2012Hugo S.E. Cruz-Garcia L. 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Med. 2014; 371: 1900-1907Crossref PubMed Scopus (17) Google Scholar). With the exception of potential diversion of ketone bodies into nonoxidative fates (see Nonoxidative Metabolic Fates of Ketone Bodies), hepatocytes lack the ability to metabolize the ketone bodies they produce. Ketone bodies synthesized de novo by liver are (1) catabolized in mitochondria of extrahepatic tissues to acetyl-CoA, which is available to the TCA cycle for terminal oxidation (Figure 1A), (2) diverted to the lipogenesis or sterol synthesis pathways (Figure 1B), or (3) excreted in the urine. As an alternative energetic fuel, ketone bodies are avidly oxidized in heart, skeletal muscle, and brain (Balasse and Féry, 1989Balasse E.O. Féry F. Ketone body production and disposal: effects of fasting, diabetes, and exercise.Diabetes Metab. Rev. 1989; 5: 247-270Crossref PubMed Google Scholar, Bentourkia et al., 2009Bentourkia M. Tremblay S. Pifferi F. Rousseau J. Lecomte R. Cunnane S. 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Chem. 1960; 235: 2450-2455Abstract Full Text PDF PubMed Google Scholar, Sandermann et al., 1986Sandermann Jr., H. McIntyre J.O. Fleischer S. Site-site interaction in the phospholipid activation of D-beta-hydroxybutyrate dehydrogenase.J. Biol. Chem. 1986; 261: 6201-6208Abstract Full Text PDF PubMed Google Scholar). A cytoplasmic D-βOHB dehydrogenase (BDH2) with only 20% sequence identity to BDH1 has a high KM for ketone bodies and plays a role in iron homeostasis (Davuluri et al., 2016Davuluri G. Song P. Liu Z. Wald D. Sakaguchi T.F. Green M.R. Devireddy L. Inactivation of 3-hydroxybutyrate dehydrogenase 2 delays zebrafish erythroid maturation by conferring premature mitophagy.Proc. Natl. Acad. Sci. USA. 2016; 113: E1460-E1469Crossref PubMed Google Scholar, Guo et al., 2006Guo K. Lukacik P. Papagrigoriou E. Meier M. Lee W.H. Adamski J. Oppermann U. 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Notably, in contrast to glucose (hexokinase) and fatty acids (acyl-CoA synthetases), the activation of ketone bodies (SCOT) into an oxidizable form does not require the investment of ATP. A reversible AcAc-CoA thiolase reaction (catalyzed by any of the four mitochondrial thiolases encoded by ACAA2, encoding an enzyme known as T1 or CT; ACAT1, encoding T2; HADHA; or HADHB) yields two molecules of acetyl-CoA, which enter the TCA cycle (Hersh and Jencks, 1967Hersh L.B. Jencks W.P. Coenzyme A transferase: kinetics and exchange reactions.J. Biol. Chem. 1967; 242: 3468-3480Abstract Full Text PDF Google Scholar, Stern et al., 1956Stern J.R. Coon M.J. Del Campillo A. Schneider M.C. Enzymes of fatty acid metabolism. IV. Preparation and properties of coenzyme A transferase.J. Biol. Chem. 1956; 221: 15-31Abstract Full Text PDF PubMed Google Scholar, Williamson et al., 1971Williamson D.H. Bates M.W. Page M.A. Krebs H.A. 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Cerebral ketone body metabolism.J. Inherit. Metab. Dis. 2005; 28: 109-121Crossref PubMed Scopus (139) Google Scholar, Robinson and Williamson, 1978Robinson A.M. Williamson D.H. Utlization of D-3-hydroxy[3-14C]butyrate for lipogenesis in vivo in lactating rat mammary gland.Biochem. J. 1978; 176: 635-638Crossref PubMed Google Scholar, Ohgami et al., 2003Ohgami M. Takahashi N. Yamasaki M. Fukui T. Expression of acetoacetyl-CoA synthetase, a novel cytosolic ketone body-utilizing enzyme, in human brain.Biochem. Pharmacol. 2003; 65: 989-994Crossref PubMed Scopus (12) Google Scholar). AACS is also highly expressed in adipose tissue, and activated osteoclasts (Aguiló et al., 2010Aguiló F. Camarero N. Relat J. Marrero P.F. Haro D. Transcriptional regulation of the human acetoacetyl-CoA synthetase gene by PPARgamma.Biochem. J. 2010; 427: 255-264Crossref PubMed Scopus (8) Google Scholar, Endemann et al., 1982Endemann G. Goetz P.G. Edmond J. Brunengraber H. 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